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Electrical Level 3 and 4 > Motor Control > Flashcards

Motor Control Flashcards

1
Q

What is the difference btwn overload protection and overcurrent protection?

A 
Overload = within normal path, damage normally thermal
Overcurrent = outside normal path, damage thermal, magnetic and arcing
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2
Q

What is the difference btwn a schematic diagram and a wiring diagram?

A

Schematic shows only electrical relationships

Wiring shows electrical and physical relationship btwn components

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3
Q

What is electrical interlocking?

A

contacts placed in the control circuit to prevent energization of the forward and reverse contacts at the same time

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4
Q

List 2 types of thermal overload relays

A

Bi-metallic

Solder pot

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5
Q

Explain on delay

A

delay occurs upon energization

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6
Q

Explain off-delay

A

delay occurs upon de-energization

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7
Q

What is meant by the term jogging?

A

Energizing the motor for a very short interval

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8
Q

Stop buttons must always be connected in ____?

A

Series

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9
Q

Start buttons must be connected in _______ ?

A

Parallel

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10
Q

List three reasons why across the line starting is the most common method used for starting AC motors?

A

High starting T
Inexpensive
Simple

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11
Q

Why might across the line starting not always be suitable?

A

High starting current causing line disturbances

High starting torque damaging to some loads

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12
Q

List three types of reduced voltage starting methods?

A

Primary resistor or reactor
Autotransformer
Wye - Delta

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13
Q

Describe the term “open transition”

A

Motor momentarily disconnects from lines during transition

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14
Q

Describe the term “closed transition”

A

Motor continuously connected to lines during transition

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15
Q

What advantage is offered by closed transition versus open transition starting methods?

A

Less line disturbances with closed transition method

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16
Q

What advantage would reactors have over resistors for reduced voltage starting?
What disadvantage?

A

Reactor are more efficient

Poorer PF

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17
Q

What advantage is gained by the autotransformer starting method compared to the primary impedance method?

A

Less line current drawn for same torque

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18
Q

Can the wye-delta starting method be used for all 3 phase induction motors?

A

No, all leads must be brought out

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19
Q

What important consideration should be observed when selecting the OLR heaters for wye-delta starting method?

A

Must be sized with phase current

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20
Q

Sync Motor Control: Explain the purpose of the relay SCA

A

Shuts down circuit if motor does not accelerate within a certain amount of time or breakdown T is exceeded.
Protects the amortisseur windings from over heating

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21
Q

Sync Motor Control: Explain the purpose of the relay PFR

A

Senses voltage induced in rotor when out of step

Decides when to apply DC to the rotor for pull-in T to occur (92-97% of sync speed)

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22
Q

Sync Motor Control: Explain the purpose of the contactor FC

A

Controls contacts which connect DC and field resistor

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23
Q

Sync Motor Control: Explain the purpose of the reactor

A

Provides voltage drops for PFR to measure

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24
Q

Sync Motor Control: Explain the purpose of the discharge resistor

A

Protects against high induced voltages in field winding when DC is applied

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25
Q

Across the line voltage starters can result in extreme _______ and cause ______ ?

A

In rush currents

Brown outs

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26
Q

Reduced voltage starters are commonly employed for 3 phase SCIMS in the ____ to ___ range?

A

5hp to 200hp

Larger motors >200hp always use RVS

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27
Q

Primary resistance starter advantages and disadvantages?

A

Advantages: Simple, reduces voltage and current, smooth accel., OK for 3, 6, 9 & 12 lead motors
Disadvantages: Power consumption, inefficient, limited capacity

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28
Q

Primary reactance advantages and disadvantages?

A

Advantages similar to resistance starter but more efficient

Disadvantages - size of inductor(s) needed.

29
Q

Another name for both resistance and reactance voltage reducing starters?

A

Primary impedance starters

30
Q

Another name for an autotransformer starter?

A

Compensator starter

31
Q

Advantages of autotransformer starters?

A

High T to motor, good for large motors
Multiple taps permit on-site selection
Efficient

32
Q

Most common taps for an autotransformer?

A

50, 65 and 80

33
Q

A wye-delta starter reduced the starting T and starting I to what?

A

1/3 or 33%

34
Q

A wye delta starter can not be used with what types of motors?

A

3 or 9 lead

35
Q

List 2 ways to removed the R from a WRIM motor controller?

A

Manually with a drum controller

Automatically with contactors

36
Q

What are the two common methods to do auto acceleration of WRIMs?

A

Definate - time acceleration using time delay relays

Speed sensing relays which sense the speed of the motor

37
Q

If some R is left in a WRIM rotor circuit, the motor will run with a higher _______ ?

A

Slip speed (reduced T)

38
Q

Below ____ % speed regulation is very unstable for WRIM speed regulator method?

A

50%

39
Q

If the out-of-step relay stops the motor, a _______ is require before restarting.

A

Cooling period

40
Q

Why is it essential that 2 of the contactor coils in a wye-delta starter be mechanically interlocked?

A

to prevent a short is both contactors closed at the same time

41
Q

Give 5 methods of braking an AC motor?

A 
Friction
Plugging
Dynamic
Regenerative
Eddy Current
42
Q

Describe plugging

A

Energizing a motor in reverse to stop it quickly

43
Q

Describe Dynamic braking

A

Slows/stops a motor by turning it into a generator.

Energy from motor and load dissipated as heat in motor windings or external resisitors

44
Q

What are the advantages of plugging?

A

Fastest deceleration
No friction parts
Suitable for large motors and severe duty

45
Q

Disadvantages of plugging?

A 
Needs a reversing contactor
Requires a zero speed switch or time delay relay
Does not work if power fails
Draws large peak currents
Stresses motor windings 
Motor is free to rotate after stopping 
With time delay method motor may reverse
46
Q

What prevents plugging from occuring before the motor is energized?

A

A lock-out solenoid

47
Q

Describe friction/electromechanical braking

A

Friction slows/stops the motor and load using a drum, band or disc brake.
Energy is dissipated as heat

48
Q

List the three types of friction brakes?

A

Drum
Disc
Band

49
Q

Most motor friction brakes are ______ applied and ______ released.

A

Spring

Electromagnetically

50
Q

How can the braking effect be varied on a friction brake system?

A

Controlling the current

51
Q

Advantages of friction brakes

A

Hold the motor from rotating when stopped
Few control components
Automatically applies if the power fails
Brake can not reverse the motor

52
Q

Disadvantages of friction brakes

A

Friction parts require maintenance
Brakes take up space
High initial cost

53
Q

Dynamic braking is also referred to as ?

A

DC injection braking

54
Q

Advantages of dynamic braking

A

Wont reverse the motor if applied to long
Gives a smooth deceleration
Braking torque is easily adjusted by varying the applied DC
No friction parts = minimal maintenance

55
Q

Disadvantages of dynamic braking

A

DC excitation required for braking
No braking effect if supply voltage removed (loss of power)
Motor can rotate after stopping
Frequent stopping can over heat the motor

56
Q

Describe regenerative braking

A

When a motor acts as a generator and energy is recovered and fed back to the supply

57
Q

Advantages of regenerative braking

A

No extra equipment may be required

No friction parts

58
Q

Disadvantages of regenerative braking

A

Only effective above sync speed

59
Q

Advantages of eddy current braking

A

The motor wont reverse if applied to long
Braking torque is easily adjusted
No friction parts

60
Q

Disadvantages of regenerative braking

A

More expensive than plugging, dynamic or regen braking
No braking can occur if the DC field is lost
Motor can rotates after stopping
Braking effect diminishes as motor slows

61
Q

Motor starters are rated in HP because?

A

They must safely interrupt the locked rotor current of the motor

62
Q

A 3 phase SCIM can be stopped using dynamic braking which?

A

Applied DC to the stator after disconnecting the AC supply

63
Q

Plugging on a 3 phase induction motor is accomplished by?

A

Reversing the phase sequence to the stator

64
Q

Starter size required for a motor load depends on?

A

Motor duty cycle

65
Q

A wye/delta starter is commonly open transition but could be closed transition by incorporating _______ into the power circuit.

A

Resistors

66
Q

Contractors and disconnect switches must be _________ rated

A

Horsepower

67
Q

A shading coil is

A

A copper ring wrapped around the core face of an AC armature, used to reduce contact chatter

68
Q

What type of motor requires 2 sets of overloads?

A

Multi speed motors

Electrical Level 3 and 4 (10 decks)

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